Optoelectronic range finders are widely used in the field of architecture, indoor decoration, and the like due to their ability to provide high measurement accuracy. Such devices typically include an emitter which emits modulated beams that may be reflected or dispersed by an object to be measured and an optoelectronic receiver. Currently, there are two methods for measuring the distance between the optoelectronic range finder and the object to be measured. One method is based on the phase measurement principle which determines the distance between the optoelectronic range finder and the object to be measured by using the phase difference between the modulated beams that are emitted and then received by the optoelectronic receiver. The second method is based on the flight time principle which calculates the distance of the object to be measured by using the flight time delay of the modulated beam when received by the optoelectronic receiver relative to when emitted from the emitter.
The measurement accuracy of such optoelectronic distance measurement devices may, however, be affected by the environment and the device itself, e.g., a temperature change in the environment and/or a temperature drift of the optoelectronic receiver. Accordingly, the phase drift due to the temperature change in the environment and/or the temperature drift of the optoelectronic receiver is currently eliminated by setting an inner reference optical path with a predetermined length within the optoelectronic distance measurement device. By way of example, U.S. Pat. No. 5,949,531 discloses a system wherein the emitting optical path is provided with a mechanical converting device for switching the modulated beams emitted from the emitter between an outer optical measurement path and an inner reference optical path whereby the modulated beams transmitted through the outer optical measuring path and the inner reference optical path may be received by the optoelectronic receiver successively to thereby generate a low frequency measurement signal and a low frequency reference signal, respectively. The measurement error of the distance measuring device may then be eliminated by the subtraction of the phases of the low frequency measurement signal and the low frequency reference signal. This method may be performed many times in one measuring process whereby the measurement signal and the reference signal alternatively enter into the optoelectronic receiver by means of the mechanical converting device. However, the currently utilized mechanical converting device may experience a large mechanical load with a result that the mechanical converting device tends to become worn and damaged during the process. Moreover, the mechanical converting device complicates the inner structure of the distance measuring device, increasing the manufacturing cost as well as the size and the weight of the distance measuring device which is not helpful for the miniaturization development of the distance measuring device.
By way of further example, U.S. Pat. No. 6,917,415 discloses a system wherein the emitting optical path is provided with a spectrometer device for dividing the modulated beams emitted from the emitter into two portions. One portion of the modulated beams is projected to the object to be measured through the outer optical measurement path and the other portion of the modulated beams are received by the optoelectronic receiver directly through the inner reference optical path. As before, a low frequency measurement signal and a low frequency reference signal are generated in the optoelectronic receiver. While this system eliminates the disadvantages of the above-described mechanical converting device, the energy of the modulated beams emitted to the object to be measured through the outer optical measurement path is decreased because a portion of the modulated beams are divided into the inner reference optical path thereby affecting the measurement ability of the device for optoelectronic distance measurement.
The emitter currently used in the optoelectronic range finder is generally a semiconductor laser which uses semiconductor material as the working medium. The semiconductor materials generally include GaAs, Cds and Zns. There are three actuation modes, that is, electronic injection, electron-beam actuation, and optical pumping. Referring to FIGS. 1 and 2, FIG. 1 shows the inner core structure of an exemplary GaAs semiconductor laser and FIG. 2 shows the package structure of an exemplary GaAs semiconductor laser which includes the working medium, resonant chamber, and pumping source. The working medium refers to the GaAs semiconductor material with a band gap itself, which is also called a PN junction. The resonant chamber generally comprises two parallel planes which are vertical to the PN junction plane, the two planes are usually used as cleavage planes of the semiconductor crystal and may be polished, and the remaining two sides of planes are relatively rough and used for eliminating the action of the laser in other directions except for the main direction. The pumping source is the current that is fed by the PN junction.
FIG. 3 shows the principle of operation for the exemplary semiconductor laser. In this regard, the semiconductor laser achieves an inversion distribution of the particle beam, and generates stimulated radiation, by feeding current into the semiconductor PN junction, and achieves the optical amplification so as to generate laser oscillation by means of the positive feedback of the resonant chamber. As the resonant chamber of the semiconductor laser is formed via the cleavage planes of the semiconductor and the cleavage planes usually have a reflectance of 35%, it is enough to induce laser oscillation. If the reflectance needs to be increased, a silicon dioxide coating may be plated on the crystal face, and then a metal coating (silver) is plated additionally thereof, thereby achieving a reflectance of more than 95%, thus the two cleavage planes may both emit laser light. In the current practice, the distance is measured by the laser beam emitted from one of the two cleavage planes which is called the main laser beam. In order to make sure that the output of the power of the main laser beam is not affected by the temperature change and voltage fluctuation, automatic compensation control for the power of the main laser beam is currently achieved by monitoring the luminous intensity of the laser beam emitted from the other cleavage plane which is called the compensating laser beam. A photodiode is presently added in the laser module to monitor the compensation laser beam, and the output power of the main laser beam is controlled to be constant by an external member. FIG. 4 shows the semiconductor laser module in the prior art, wherein the laser module includes a laser diode (LD) and a photodiode (PD) wherein the two cleavage planes of the laser diode respectively emit the main laser beam and the compensation laser beam while the photodiode is used to monitor the luminous intensity of the compensation laser beam.